1. Field
The disclosure of the present application generally relates to the fabrication of printed circuit boards, and more particularly to entry sheets for drilling holes in printed circuit boards.
2. Description of the Related Art
Holes are drilled in electrical interconnect systems such as printed circuit boards (PCBs) for various purposes, including to accommodate component leads, interconnect different layers of PCBs, etc. Several problems are associated with drilling holes in PCBs. Entry burrs, the waste substance that is removed to create holes in PCBs, can cause defects in the holes, such as, for example, blowholes and voids, by rolling into barrels of the holes. Drilling can also cause damage to top surfaces of a stack of PCBs, for example, when the pressure foot of a drilling machine impacts the PCBs, thereby damaging the top surfaces of the PCBs. Drill accuracy is also factor to consider during drilling. Ideally, for example, a hole at the top of a stack of PCBs is aligned to a corresponding hole at the bottom of the stack of PCBs. The top hole and the bottom hole can be misaligned for various reasons, including, for example, due to inaccurate positioning of drill spindles relative to the drill axis upon entering the top surface of PCBs. The position of the drill point upon entering the top of the stack of PCBs can affect the location of the hole at the bottom of the stack. The closer to the center of the top hole the drill enters the sack of PCBs, the better the location of the corresponding bottom hole. Further, the drill spindle in some situations may be deflected from its true position, thereby adversely affecting the position of the bottom hole, and decreasing drilling accuracy.
To address some of the aforementioned drilling problems, entry boards are typically provided over PCBs as intermediate layers through which drill spindles advance prior to drilling various layers of PCBs. However, some entry boards are only effective for certain drill diameters. Others fail to provide a satisfactorily solution to all of the major issues associated with drilling. For example, although some entry boards generally improve drilling accuracy, the quality and location of the holes can benefit from even more improved accuracy. The present application provides improved systems and methods that attempt to effectively address these and other issues, for example, without limitation, by minimizing entry burring, protecting PCBs from damage, and/or improving drilling accuracy, etc.